Modern communication networks, such as the Public Switched Telephone Network (PSTN), are used to transmit voice and data signals around the world. For example, FIG. 1 illustrates a conventional communications network, including the PSTN 100. As shown in FIG. 1, a central office (CO) 110 of a local telephone company may provide users or subscribers 120a–c with access to the PSTN 100. The portion of the network between the CO 110 and the users 120a–c may be referred to as the local loop 130. The local loop 130 may include a series of transmission line cables 140a–c which may be carried via telephone poles and/or buried underground between the CO 110 and the users or subscribers 120a–c. The design and operation of the PSTN 100 and the CO 110 are well known to those having skill in the art and need not be described further herein.
The transmission line cables 140a–c used in the local loop 130 may each include a plurality of twisted wire pairs, known as POTS (Plain Old Telephone Service) lines. These wire pairs can have substantial capacitance, which may result in a change in impedance with the length of the transmission line. As is well known in transmission line theory, an improperly matched transmission line and load impedance may result in only part of a transmitted signal to be absorbed, with the remainder being reflected back on the twisted pair, which may result in interference on the line and thus signal distortion and/or degradation. As these capacitance effects may increase with transmission line length, they may directly impact the voice band (300 Hz to 3000 Hz) such that higher voice frequencies may be subjected to greater loss or attenuation. As the length of the transmission line is increased beyond 18,000 feet, this attenuation may pose a significant obstacle to voice transmission.
FIG. 2 illustrates a conventional local loop, including a transmission line cable between a CO 210 and a user 220. Referring to FIG. 2, load coils 230a–c are inductors which may be placed on the transmission line 240 to compensate for the capacitive effects at increased transmission line lengths. The load coils 230a–c may be inserted in series with the wire pairs of the transmission line 240 at specific intervals (such as every 6000 feet), so that the known capacitance of the wire pairs may be balanced by the inductance of the load coils 230a–c to maintain a predetermined line impedance. Thus, the effective capacitance of the loop may be reduced, balancing the attenuation across the voice band. As a result, signal reflection may be lowered and voice quality may be improved.
A potential drawback of load coils is their effect on broadband data transmission, such as DSL (Digital Subscriber Line). Since each load coil may appear as extremely high impedance to high-frequency data transmission, DSL and other broadband connections may not be effectively deployed on loaded circuits. In other words, the load coils act as low-pass filters, so that high frequencies cannot pass through the coils. As such, when a user or subscriber wants high frequency service, each and every load coil located on the transmission line between the CO and the user must be “unloaded” or bypassed from the wire pair connected to the particular user.
FIG. 3 illustrates a conventional load coil enclosure installed on a transmission line section. As shown in FIG. 3, the transmission line section 300 includes a load coil enclosure 320 and a transmission line 340. The load coil enclosure 320 includes a plurality of load coils, each of which is connected to a respective one of the plurality of twisted wire pairs included in the transmission line 340 through a splice closure 350. The splice closure 350 is a terminal casing designed to cover the area of the transmission line 340 where the plurality of wire pairs have been exposed for repair, maintenance, and/or installation of network elements. Although the load coil enclosure 320 and splice closure 350 are illustrated as mounted on a telephone pole 360, such enclosures may also be mounted in cabinets, underground manholes, or the like.
In order to bypass a load coil, the location of the load coil enclosure in the outside environment may need to be determined. After gaining access to the load coil enclosure, the specific wire pair servicing the user may need to be separated from the potentially hundreds of wire pairs typically found in transmission line cables so that the corresponding load coil may be bypassed by splicing the wire pair around the load coil. The cable may then need to be recovered with metallic and plastic sheaths, pressurized, and tested for leaks. Alternatively, a user may require that a disconnected load coil be re-connected to the wire pair in a similar manner. In either case, it may typically take two technicians eight hours or more to complete the splicing operation for each load coil on a user's wire pair. Further, bypassing or re-connecting these coils may require coordination between engineering teams and construction crews, resulting in service delays to the customer.
A load coil enclosure that includes load coils and switches within the same housing such that each load coil can be connected or disconnected from a wire pair using a corresponding switch rather than physically removing each coil from the wire pair is discussed, for example, in U.S. Pat. Nos. 5,929,402 and 6,281,454 to Charles et al. However, in order to use such switchable load coil enclosures, a telephone company may be required to replace load coil enclosures that are currently in use. As load coils have been used on transmission lines since the late 1960s, thousands of load coil enclosures are already in place today. To replace all of these existing enclosures with the switchable load coil enclosures of Charles et al. may involve a tremendous cost to the telephone companies, and as such, may be undesirable.
In view of the foregoing, it may be desirable to provide a solution that quickly allows technicians to add or remove load coils from transmission lines without requiring time-consuming splices, service delays, and/or replacement of existing load coil enclosures.